#include <iostream>
#include <cfloat>

bool hmean(double a, double b, double *);

int main()
{
    double x, y, z;
    std::cout << "Enter two numbers: ";
    while(std::cin >> x >> y)
    {
        if(hmean(x, y, &z))
            std::cout << "harmonic mean of " << x << " and " << y << " is " << z << std::endl;
        else
            std::cout << "One value should not be the negative of the other - try again.\n";
        std::cout << "Enter next set fo numbers <q to quit>: ";
    }
    std::cout << "Bye.\n";
    return 0;
}

bool hmean(double a, double b, double * ans)
{
    if(a == -b)
    {
        *ans = DBL_MAX;
        return false;
    }
    else
    {
        *ans = 2.0 * a * b / (a + b);
        return true;
    }
}
